gtk.css

/* write this into your .config/gtk-3.0/gtk.css, you might modify the colors a bit to look better with your destkop */

#whiskermenu-button {
  background-color: transparent;
}

#whiskermenu-window * {
  border: 0px #000 solid;
  outline: none;
  overflow: auto;
}

window#whiskermenu-window box {
  padding: 5px;
}

@keyframes appearFromBottom {
  from {
    opacity: 0;
    transform: translateY(20px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

#whiskermenu-window {
  background-color: rgba(28, 28, 28, 0.9);
  color: #ccc;
  border-radius: 0px 15px 0px 0px;
  padding: 20px;
  opacity: 0;
  transform: translateY(20px);
  display: none;
  animation: appearFromBottom 0.5s ease-out forwards;
}

#whiskermenu-window scrollbar {
  background: transparent;
}

#whiskermenu-window entry {
  background-color: rgba(28, 28, 28, 0.9);
  color: #7b7b7b;
  border-radius: 10px;
}

#whiskermenu-window entry:focus {
  background-color: rgba(28, 28, 28, 0.9);
  color: #7b7b7b;
  border-radius: 10px;
  border: none;
  outline: none;
  box-shadow: none;
}

#whiskermenu-window button {
  background-color: rgba(25, 25, 25, 1.0);
  color: #9a9a9a;
  margin: 0px;
  margin-right: 0px;
  border-radius: 6px;
  background-image: none;
  font-weight: normal;
}

#whiskermenu-window treeview {
  background-color: rgba(28, 28, 28, 0);
  color: #ccc;
  background-image: none;
  outline-color: #F00;
}

#whiskermenu-window button:focus,
#whiskermenu-window button:hover,
#whiskermenu-window button:active,
#whiskermenu-window button:selected,
#whiskermenu-window treeview:selected,
#whiskermenu-window treeview:hover,
#whiskermenu-window treeview:active,
#whiskermenu-window treeview:focus {
  background-color: #2B2D23;
  color: #bdbdbd;
  transition: 200ms;
}

#whiskermenu-window button:active {
  background: #2B2D23;
}

#whiskermenu-window treeview:focus {
  background: transparent;
}

#whiskermenu-window iconview {
  background: transparent;
  border-radius: 4px;
  font-family: "Nunito";
  color: #9a9c91;
  font-size: small;
  padding-top: 2px;
}

#whiskermenu-window iconview:hover,
#whiskermenu-window iconview:selected {
  background: rgba(255, 255, 255, 0.086);
}

#whiskermenu-window .frame:nth-child(1),
#whiskermenu-window .frame:nth-child(2),
#whiskermenu-window .frame:nth-child(3),
#whiskermenu-window .frame:nth-child(4),
#whiskermenu-window .frame:nth-child(5),
#whiskermenu-window .frame:nth-child(6) {
  background: rgba(48, 48, 46, 0.35);
  border-radius: 8px;
}

window#whiskermenu-window box widget {
  color: transparent;
}

#whiskermenu-button {
  background-color: #26221e;
  color: #FFFFFF;
  font-family: "Ubuntu Bold", sans-serif;
  font-size: 14px;
  padding-left: 15px;
  padding-right: 20px;
  border-radius: 10px;
  border: 1px solid #78695d;
  transition: background-color 0.1s ease;
}

#whiskermenu-button:hover {
  background-color: #2b2b2b;
  color: #FFFFFF;
  font-family: "Ubuntu Bold", sans-serif;
  font-size: 14px;
  padding-left: 15px;
  padding-right: 20px;
  border-radius: 10px;
}

#whiskermenu-button:active {
  background-color: #a1a1a1;
}